1. Field of the Invention
The present invention relates to a wireless communication apparatus and method for data communications where the capacity of transmission is extended using a spatial multiplexing communication scheme (multiple input multiple output; MIMO) in combination with a transmitter having a plurality of antennas, and to a computer program applicable to the wireless communication apparatus and method. Specifically, the present invention relates to a wireless communication apparatus and method for data communications which receives packets transmitted after beamforming processing with cyclic delay diversity (CDD), and to a computer program applicable to the wireless communication apparatus and method.
More specifically, the present invention relates to a wireless communication apparatus and method where suitable synchronous timing is obtained based on correlation processing of preamble from packets transmitted with different delay amounts for respective transmission branches, and to a computer program applicable to the wireless communication apparatus and method. Still more preferably, the present invention relates to the present invention relates to a wireless communication apparatus and method where a dynamic range to be indispensable for receiving packets transmitted with different delay amounts for respective transmission branches is diminished, and to a computer program applicable to the wireless communication apparatus and method.
2. Description of the Related Art
Wireless networks have become a focus of attention, as systems free from wiring used for known wired-communication methods. EE (The Institute of Electrical and Electronics Engineers) 802.11 and/or IEEE 802.15 can be named, as an ordinary standard relating to the wireless network. For example, where IEEE 802.11a/g is used, an OFDM (Orthogonal Frequency Division Multiplexing) modulation method which is one of multi-carrier systems is used, as an ordinary standard of wireless LANs.
When IEEE 802.11a/g standards are used, a modulation method achieving a communication speed of 54 Mbps at the maximum is supported. However, a next-generation wireless-LAN standard capable of achieving a higher bit rate has been demanded. The MIMO (Multi-Input Multi-Output) communication draws attention as one of technologies for realizing high-speed wireless communication. For instance, IEEE 802.11n (TGn), an extended standard of IEEE 802.11, employs the OFDM_MIMO communication system.
MIMO is a communication system that realizes spatial multiplexing streams in each of a transmitter and a receiver provided with a plurality of antenna elements.
The transmitter performs space-time coding on a plurality of transmitted data and distributes the multiplexed data to a plurality of transmission antennas, followed by transmitting the data to a channel. On the other hand, the receiver receives signals from the transmission branch by a plurality of receiving antennas through the channel and performs space-time decoding on the received signals to divide them into a plurality of transmission data. Thus, the receiving branch obtains the original data without any cross talk between streams. The MIMO communication system can attain an increase in communication speed by extending the capacity of transmission depending on the number of antennas without extending the range of frequencies. In addition, the MIMO communication system has good frequency utilization efficiency because of using spatial multiplexing. MIMO is a communication mode using a channel characteristic, and it is different from a mere transceiver adaptive array.
In MIMO communications, a transmission weight matrix and a reception weight matrix are calculated using channel-matrix H, respectively. Here, the transmission weight matrix is provided for carrying out spatial multiplexing of transmission streams from a plurality of transmission branches in the transmitter. The reception weight matrix is provided for carrying out spatial separation of the spatial multiplexing signals into a plurality of original streams in the receiver. The channel-matrix H is the numerical matrix using channel information corresponding to a pair of transmission/reception antennas as an element. The term “channel information” used herein is a said here is a transfer function having a phase and amplitude as components. Usually, a channel matrix can be presumed by carrying out a frame exchange sequence including training series constructed of known reference symbols for exciting the channel matrix between the transmitter and the receiver.
In the MIMO communication, an unexpected beam may be formed when identical or similar signals are transmitted through different space streams. For this reason, in IEEE 802.11n, a method of transmitting a time-difference signal from each transmission antenna, so-called cyclic shift or cyclic delay diversity (CDD), is employed.
For example, a wireless communication apparatus for correctly receiving a MIMO_OFDM signal has been disclosed in Japanese Published Patent Application No. 2007-221187. In this method, the usual synchronization acquisition processing is performed using a field for synchronization acquisition in a preamble and a signal is then detected as a MIMO signal to which the addition of a cyclic shift signal is performed between streams, allowing synchronous timing to be adjusted based on the cyclic shift thereof to correctly receive the MIMO_OFDM signal.
In wireless communications, it is noted that a preamble having a repetition of a known training sequence is generally added to the head of a packet. Thus, a receiver performs synchronous processing using such a preamble. Specifically, if the receiver finds a packet by detecting the preamble, the receiver performs subsequent processing, such as the check of precise receiving timing, frequency-offset removing operation, and optionally normalization of received signal electric power (setup of automatic gain control (AGC); AGC gain). Then, the valid symbol section of an OFDM symbol is extracted to feed a received signal to a fast Fourier transform (FFT).
However, a plurality of correlation peaks appears when performing CDD in a MIMO communication system and performing a synchronous processing by a receiver (namely, when applying different delay amounts to the respective packets to be transmitted from two or more transmission antennas). Therefore, if the synchronous timing is obtained by averaging or weighted averaging of the correlation value of the receiving branch, erroneous detection probability may be increased depending on a channel or receiving environment.
It is surmised that the signal level received with two or more antennas by the receiver end has big variation under the influence of multipath fading. Therefore, if each of the reception branches is adapted to the minimum or maximum reception gain, an extremely large dynamic range will be necessary for the subsequent reception operation.